I would strongly suggest doing your own research and then speaking to a surgeon about which options are the best for you. There's no point getting a strong inclination towards one specific donor site only to find out that it is not ideal for you. But I do think it's important to know what all is out there. The wiki here is a good starting point. There are also phalloplasty-specific information sites out there.
To answer your questions, ALT or MLD have no real size restrictions other than not being able to exceed the length of the donor area (so the length of the thigh for ALT and the length of the side for MLD). Unless it fits your body (ie. you are very tall), I think you'll encounter a lot of push back from a surgeon about requesting anything above 6". It's riskier to go bigger, as the phallus has to have more support and has to work that much harder to re-establish its blood supply. And you'd also be one size all the time. It sounds great for sex, but it is not so great for regular life.
